1. Field of the Invention
This invention relates generally to medical devices and especially intravascular catheters designed to operate with respect to occlusions within a blood vessel. More particularly, this invention relates to intravascular catheters having the ability to fracture an occlusion sufficiently for allowing a guide wire to pass through the occlusion within the natural lumen of the blood vessel.
2. Background
Medical science has long sought effective treatments for disease conditions involving stenosis (narrowing or obstruction) of the lumen (interior passage of the artery) of an artery. This condition, known generally as an occlusion, is found in patients suffering from atherosclerosis (accumulation of fibrous, fatty or calcified tissue in the arteries). An occlusion can manifest itself in hypertension (high blood pressure), ischemia (deficiency of circulation), angina (chest pain), myocardial infarction (heart attack), stroke, or death. An occlusion may be partial or total, may be soft and pliable or hard and calcified, and may be found at a great variety of sites in the arterial system including the aorta, the coronary and carotid arteries, and peripheral arteries.
Of particular interest to cardiac medicine are the often disabling or fatal occlusions occurring in the coronary arteries (arteries supplying the heart). Traditionally, coronary artery occlusions have been treated by performing coronary bypass surgery, in which a segment of the patient's saphenous vein is taken from the patient's leg and is grafted onto the affected artery at points proximal (upstream) and distal (downstream) to the occluded segment. The bypass often provides dramatic relief. However, it entails dangerous open chest surgery and a long, painful, costly convalescence in the hospital. Moreover, with the passage of time, the bypass patient's saphenous vein graft can also become occluded. If the patient has another saphenous vein, a second bypass procedure may be performed, once again entailing open chest surgery and prolonged hospitalization. Thereafter, if the underlying atherosclerotic disease process is not controlled, the prognosis is dismal.
Newer, minimally invasive procedures are now preferred in the treatment of arterial occlusions. These procedures use a catheter, a long, thin, highly flexible device which is introduced into a major artery through a small arterial puncture made in the groin, upper arm, or neck and is advanced and steered into the site of the stenosis. At the distal end of the catheter, a great variety of miniature devices have been developed for operating upon the stenosed artery.
The more popular minimally invasive procedures include percutaneous transluminal coronary angioplasty (PTCA), directional coronary atherectomy (DCA), and stenting. PTCA employs a balloon to mechanically dilate the stenosis. In PTCA, a steerable guidewire is introduced and advanced under fluoroscopic observation into the stenosed artery and past the stenosis. Next, a balloon-tipped catheter is advanced over the guidewire until it is positioned across the stenosed segment. The balloon is then inflated, separating or fracturing the atheroma (stenosed tissue). The hoped-for outcome is that, over time, the lumen will stay open.
In directional coronary atherectomy, a catheter containing a cutter housed in its distal end is advanced over the guidewire into the stenosed segment. The housing is urged against the atheroma by the inflation of a balloon, so that part of the atheroma intrudes through a window in the side of the housing. Under fluoroscopic observation, the cutter is used to shave away the atheroma. The shavings are collected in the nosecone of the housing and withdrawn along with the catheter.
Stenting is a procedure in which a wire framework, known as a stent, is compressed and delivered a balloon catheter. The stent is positioned across the stenosed segment of the artery. The balloon is inflated, dilating the stent and forcing the stent against the artery wall. The hoped-for outcome is that the stent will hold the arterial lumen open for a prolonged period. Frequently, a stent is placed in an artery immediately following PTCA or DCA.
It must be noted, however, that the aforementioned catheters are "over-the-wire catheters." These catheters depend on the guidewire, which typically has a tiny bent portion at its distal end for steering. Over-the-wire catheters cannot be positioned adjacent the stenosis until the guidewire has been advanced across the stenosed arterial segment. Thus, where the occlusion is too severe to be crossed by a guidewire or where there is not enough room for the balloon, cutter, or stent delivery catheter, neither PTCA nor DCA nor stenting can be done. Unfortunately, the occlusion often contains extremely hard, calcified tissue and presents an impenetrable barrier to the guidewire. Even a less than total occlusion may contain complex structures which divert or trap the steering end of the guidewire. Thus, the guidewire might not completely cross the occlusion, but become diverted into the subintimal space between the intima and the atheroma or become buried in the atheroma. In either case, the guidewire cannot be positioned across the stenosis to guide a balloon or cutting element. In such cases, bypass surgery may be necessary with the associated cost, risks, and recovery period.
Thus, in patients suffering from severe or total arterial occlusion, it is preferable to do what has been difficult or impossible in the past: to open the severely or totally occluded artery itself, rather than by performing a bypass. If a guidewire and working catheter can be passed through or around the atheroma, the severe or total occlusion can be treated by PTCA, DCA, stenting, site-specific drug delivery or a combination of these proven therapies.
It would be advantageous to find and open a path of low resistance, either through or around the atheroma. Of course, this must be done without perforating arterial wall. Clearly, the serious consequences of penetrating the arterial wall must be avoided at all costs. The physician will not use a system which would be unsafe and no patient would want a physician to use such a system. Therefore, any solution to the problem of finding and creating an opening through or around the atheroma must be safe and in many instances include a system of guidance for the device that would find and open such an occlusion.
There has been a long felt need to provide a reliable system of guidance for such a device. As understood by those in the art, the device must travel a crisscrossing, often maze-like structure before it even gets to the occlusion. Then the occlusion itself is often a maze like structure. Attempting to cross such an occlusion without reliable guidance is dangerous. For example, it is easy to dissect the tissues of the arterial wall instead of the occlusion, thereby creating a false lumen and possibly perforating the artery. If blood escapes the artery and accumulates in the pericardial space, it will compress the heart, requiring emergency intervention to avert heart failure and death.
One guidance system which has been used in conjunction with coronary catheterization is biplane fluoroscopy, wherein the interventionist observes two flat real-time x-ray images acquired from different angles. Biplane fluoroscopy, however, is unreliable, costly, and slow. Delay is unacceptable, for it contributes to trauma and stress and creates opportunities for complications and failures of technique.
Recently, promising optical systems have been disclosed for imaging an occlusion through a catheter placed in the artery. One such system is Optical Coherence Tomography (OCT). In OCT, a beam of light carried by an optical fiber illuminates the artery interior. In a radar-like manner, light reflected back into the fiber from features inside the artery is correlated with the emitted light to capture the depth as well as the angular separation of those features. The features are displayed graphically in two or three dimensions through the use of a suitably programmed computer.
The beam in OCT is swept by mechanical rotation or movement of optical components in the catheter, or by optical switching devices which select one of several fibers through which to perform measurements. The rotation is encoded, or the switching pattern recorded, for reconstructing angular information about the artery interior. For example, a beam splitter may be placed between the light source and the catheter fiber to produce a reference beam which is directed to a reflector at a known distance. The catheter beam and the reference beam are recombined as they return. When the paths traveled by the two beams are of equal optical length, interference fringes are observable in the combined beam. Since the lengths of the reference path and the catheter fiber are known, the distance from the fiber end to a particular reflective feature within the artery can be inferred. In OCT and related methods, signals may also be impressed upon the light beam to facilitate the measurement of distance or the detection of motion of objects relative to the fiber end. By means of OCT or other similar optical methods, imaging capability can be incorporated into an intravascular catheter or guidewire.
However, while superior imagery alone is of diagnostic interest, effective intervention for severe occlusive arterial disease is what is truly desired. Even with improved guidance, there persists a long felt need for working elements which are capable of opening a path through or around an arterial occlusion at low risk of perforating the artery. What is needed is an intravascular catheter system for the effective treatment of the severely occluded artery and, in particular, the totally occluded artery. What is especially needed is a therapeutic working element which allows the physician to mechanically fracture an occlusion or to separate the occlusion from the intimal surface, but which is operable in a manner unlikely to perforate the adventitia.